Myths about getting better gas mileage debunked

There are plenty of legitimate ways to stretch your mileage: slow down, keep tires at proper inflation, avoid quick acceleration, don't pile luggage on a roof rack, use a properly fitted gas cap.

But at this time of skyrocketing gasoline prices, several myths are circulating. The claims were tested by car experts at the Automobile Club of Southern California and Consumer Reports magazine.

Fill up in the morning

The theory is that gas expands in warm weather, so if you visit the filling station early in the day when the temperature is cooler, you get denser fuel that contains more energy per gallon. In other words, fill up when it's cool to get more bang for your gasoline buck.

There's nothing wrong with the theory, except that it doesn't apply in this situation. That's because gas station storage tanks are so well-insulated these days that outside temperature has little effect.

"The key thing is the temperature of the gas when the tanker truck leaves it at the gas station," said Steve Mazor, manager of the auto club's research center. "If it's hot, it will stay hot, and vice versa."

No matter what time of day you fill up.

Change your air filter

The test team at Consumer Reports was sure that a car would get better mileage with a clean rather than a dirty air filter.

"It stands to reason that if the air is allowed to flow freely, it would result in better fuel economy," spokesman Douglas Love said.

To do the test, the team used duct tape to partially cover the air intake, simulating a dirty filter. And the result?

"We were surprised to find out it didn't much matter," Love said. "The mileage was about the same."

So, testers added a bit more tape and then some more. Short of blocking off all the air, they got the same results.

"We found that the onboard computers that adjust the fuel mixtures on recent cars did a surprisingly effective job," he said.

The auto club ran a test with clean and dirty fuel filters on several different cars. Results were similar.

"There was even one test where the mileage got a little bit better -- maybe 1% or 2% -- with the dirty filter," Mazor said.

But before you count the money you can save in air filters, there is a price to pay.

"The trade-off was that carbon monoxide emissions went up," he said.